Can ecosystem-scale translocations mitigate the impact of climate change on terrestrial biodiversity? Promises, pitfalls, and possibilities

Rationale for the proposed approach

Species interactions are essential to the functioning of ecosystems. However, in the context of climate change and biodiversity crises, understanding the full range of community interactions that are needed to recreate a functioning ecosystem in a potential receptor area is not realistic¹⁴. It follows that ecological engineering through manual planting or regeneration through the seedbank, as is applied in classical restoration programmes¹⁵, is not a practical solution for conserving functioning above- and below-ground communities of ecosystems threatened by climate change. On the contrary, the translocation of topsoil, vegetation and all the communities they contain could represent a much-needed shortcut where all components and interactions of an ecosystem are potentially preserved.

Existing examples of ecosystem translocation

Under the terms habitat translocation, community translocation, vegetation translocation or transplanting, ecosystem translocation has been applied at a small scale to conserve particular plant species or communities impacted by proposed land development¹⁶ but also to test the robustness of plant communities in climatically challenging conditions^11,17. A number of earlier translocation attempts reviewed by Good et al.¹⁸ showed that with appropriate preparation of the receiver sites, translocation of turves produced satisfactory results in terms of plant survival and conservation of community composition for a variety of plant community types. In the mining industry, a similar process called vegetation direct transfer (VDT) has been applied for 30 years as part of mine restoration programmes^19–21 sometimes translocating large areas. For example, a total of 75 ha of native grassland, shrubland and low canopy forest have been translocated within the Stockton Mine (New Zealand) in the past 30 years^22–24. The method used consists of (i) cutting pieces of land comprising soil, roots and vegetation over a 1–3 m² area and a depth of 30 cm or more using a digger, (ii) transporting these sods by truck to a new area and (iii) reconstituting the ‘jigsaw’ with minimum gaps between the sods to recreate a continuous habitat (Figure 1). Many studies have highlighted the positive outcomes of such translocations, including: the conservation of plant and beetle communities, soil functions and microbial activity after wetland translocation^25,26; the maintenance of soil structure and fertility in translocated grassland; rtebrates (e.g. carabid beetles, weta) and plant species (beech) from shrubland and tussock wetlands²²; the preservation of biodiversity and biomass in soil fauna²⁷ and microbial communities²⁸ from alpine vegetation; and the conservation of habitat for birds and invertebrates from translocated forest areas²¹. There is also an extensive body of work, including peer-reviewed articles^21,24,26,29 and reviews^18,19,21, but also internal reports and conference presentations (see Supplementary material), most of which point to the conclusion that VDT promotes faster ecosystem recovery than do other conventional restoration techniques such as replanting or hydroseeding.

Figure 1. Vegetation direct transfer of native alpine forest at the Stockton opencast coal mine (New Zealand).

Clockwise: pieces of land comprising vegetation, roots and soil are cut; soil and vegetation sods are loaded on a truck and transported to the receptor site; the landscape jigsaw is then reconstituted with minimum gaps between the sods to recreate a continuous habitat. Credits: Stéphane Boyer and Solid Energy New Zealand Ltd.

Ecosystem-scale translocation - promises

Could ecosystem-scale translocation represent a practical and economic translocation method, by which all or most immobile, slow moving and low-dispersal elements of a terrestrial ecosystem are moved together as a functioning community of above- and below-ground organisms? While no taxonomic group is particularly targeted by EST, it is likely that plant, microbial and fungal communities as well as most invertebrates and small vertebrates will directly benefit, while large and mobile terrestrial vertebrates will probably vacate the site due to disturbance during the translocation process. Therefore, EST constitutes a complementary approach to single-species conservation translocation programs, which suffer from a significant taxonomic bias towards larger animals. Indeed, almost 60% of all animal translocation efforts to date have been for large and charismatic species of birds and mammals⁷, despite these taxa comprising only c. 1% of all known animal species³⁰. Although translocation distances and the sizes of the areas translocated are usually minimised for economic and practical reasons in a restoration context (Figure 2), this process could be scaled up for the purpose of ESTs where the aim is to mitigate the impact of climate change on biodiversity in highly-valued ecosystems. The aim of EST should be to preserve a functioning representative subset of an ecosystem of sufficient size to ensure its long-term survival and functioning. Capturing most ecosystem functions is likely to require the translocation, in stages, of fairly large areas. However, the majority of the current protected areas globally are less than 1,000 ha and places such as Barro Colorado in Panama (1,500 ha), the Bosavi Crater in Papua New Guinea (1,250 ha), or Darwin Island in the Galapagos (110 ha), are good examples of relatively small areas with significant biodiversity interest.

Figure 2. Conceptual diagram of the size of the area translocated in relation to translocation distances.

The orange area corresponds to typical past and current habitat translocation projects, with dots corresponding to published studies (see Supplementary material S1 for a complete list). The green area represents an ideal combination of size and distance for ecosystem-scale translocation, with darker green more suitable, although more difficult to put in place.

Ecosystem-scale translocation - pitfalls

Only a handful of translocation studies have been reported in the scientific literature (Supplementary material) and the extent to which biodiversity conservation was measured in these studies is very limited. More targeted research is therefore required to assess to what extent EST can conserve biodiversity at a range of levels including species, genetic and functional diversity, as well as species interactions and ecosystem functions. Critical factors and recommendations required for habitat translocation include similarity between the environmental conditions of the donor and receptor sites, the translocation technique, and appropriate management of the translocated habitat³¹. In particular, the nature of the bedrock strata as well as the geochemistry, hydrology and precipitations at the receptor site will have a strong influence on the translocated topsoil. Geology can be measured and matched between donor and receptor sites, while the range of possible changes in fine-scale hydro-dynamics may be much more difficult to predict as climate changes³². One additional and outstanding issue is the selection of a candidate receptor site where the ‘graft’ will not replace communities which are themselves of high-value, possibly leading to the extinction of local species¹⁴ or negatively impacting the functioning of surrounding ecosystems^6,9. For example, some phytophagous insects could rapidly become invasive when encountering new plant species within their distribution range³³. Conversely, the spillover of existing invasive species from areas surrounding the receptor site could negatively impact the graft itself. This is possibly of less concern because species invasion is less likely to happen when translocating complete and functioning ecosystems where most or all niches are occupied and, consequently, where the lack of empty niches make ecosystems more resistant to the establishment of invasive species³⁴. Supposedly, invasion risk could also be reduced if the receptor site is located, within areas where species composition does not differ dramatically from that of the donor site. Nevertheless, it is important to note that any negative impacts are likely to be difficult and costly to reverse after translocation and a thorough risk assessment would be required to ensure that translocated species do not affect the surrounding areas after translocation.

The cost involved in the translocation of ecosystems is likely to constitute an important limitation. Because only few examples exist, it is difficult to provide a definite price per unit area. One comparative study by Simcock et al.²² showed that translocation in a mine rehabilitation context was seven times more costly than transporting bulldozed soil and vegetation, but twice cheaper than rehabilitation through nursery cultivation followed by manual replanting. This study was based on the translocation of a 0.6 ha of native forest over a few hundred meters distance and most of the additional cost was from transportation between donor and receptor area as trucks could only carry a single layer of sods (i.e. 30 to 50% of full capacity)²². Translocation of larger areas over much longer distances, as required for EST (Figure 2), is likely to be much more costly.

Potential receptor sites

In addition to a deep understanding of the risks and potential side effects, the application of EST also requires carefully selected receptor sites. Suitable receptor sites should have very low intrinsic biodiversity value because they will be replaced by EST and whatever communities were present will disappear. Such potential sites could be selected in agricultural land that has been marginalised or abandoned due to the erosion of the fertile soil layer and the loss of essential soil nutrients following overexploitation or misuse³⁵. It has recently been estimated that 25% of all terrestrial habitat is highly degraded³⁶, with one hectare of productive land lost globally every 7.67 seconds (www.irri.org). There is growing interest in using such marginal and degraded land to cultivate biofuel crops^37,38. However, this land could alternatively be used for biological conservation. When soil is lost or degraded to the extent where growing crops is not possible, the land could still be suitable for receiving EST, because the latter includes topsoil and its associated ecosystem functions. With human-degraded soils scattered across all continents³⁷ potential receptor sites could be available in a range of climate zones.

Assisted colonisation of single species includes moving organisms to habitats with more favorable future climatic conditions⁵. The timing of assisted colonisation is therefore crucial because those performed before the recipient region is predicted to be climatically suitable are likely to fail³⁹. Determining the “window of opportunity” – a scenario where an ecosystem is still viable at its original location, and where there is an existing suitable receptor site - is likely to prove difficult and will require active adaptive management⁴⁰. To alleviate the above limitation, EST could be applied to move ecosystems from climatically unstable areas to areas where the climate is projected to remain stable despite global changes⁴¹. The climate stability index proposed by Iwamura et al.⁴² can be used in this context. This index compares observed temperature, precipitation, cloud cover and vapour pressure in 1997–2002 to projected values in 2047–2052. Human-degraded soils that occur in climatically stable areas could offer prime locations to receive ESTs and could receive them from regions of high conservation value (e.g. biodiversity hotspots) currently located in unstable climatic areas.

An illustration of the potential for EST

Using a highly conservative GIS approach (see Method section), we identified a range of potential source and receptor areas at a global scale. The former were located mainly in North America and North Eurasia as well as over the Amazonian forest (Figure 3A), while a large majority of potential receptor areas were located in Africa, with smaller areas also identified in America, the Middle East, Asia and Australasia (Figure 3B). The suitability of these source and receptor areas is largely dependent on the accuracy of current climatic predictive models, which are constantly improving⁴³. Also, climatic stability thresholds and criteria used for selecting highly valuable ecosystems or degraded land can vary greatly. In a similar way to conservation programmes, which often struggle to cross borders⁴⁴ due to different policy in neighbouring countries and a lack of global legislation, EST is highly unlikely to be applied in a cross-border situation (where an ecosystem with high conservation value would be translocated from one country to another). Therefore, we identified countries where large areas of both source and receptor sites were present (Figure 3C). They include Madagascar, USA (California), Mexico, Turkey, Nigeria, Cameroon, South Africa, Vietnam, Thailand, Myanmar and Indonesia (Table 1). These countries may therefore be ideal candidates to trial EST.

Figure 3A. Global map of highly valued areas that could be considered as potential source for ecosystem-scale translocation.

Ideal source areas are those where high value (blue lines) overlaps with low climatic stability (in pink).

Figure 3B. Global map of potential candidate receptor areas for ecosystem-scale translocation.

Ideal sites are those where high soil degradation (red lines) overlaps with high climatic stability (in purple).

Figure 3C. Global map of potential source (in blue) and receptor areas (in red) as defined in the legend of Figure 3A & Figure 3B.